Problem: Which of these numbers is prime? ${18,\ 41,\ 45,\ 77,\ 93}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 18 are 1, 2, 3, 6, 9, and 18. The factors of 41 are 1 and 41. The factors of 45 are 1, 3, 5, 9, 15, and 45. The factors of 77 are 1, 7, 11, and 77. The factors of 93 are 1, 3, 31, and 93. Thus, 41 is a prime number.